In the preferred application of the invention to an outside swimming pool cover, various techniques are already known.
A first technique consists of using a simple sheet which comprises an impermeable layer held at the surface of the water by floats or by cords provided from place to place; another technique derived from the previous one consists of incorporating in this sheet a sort of air bag by means of very thin bubbles which ensure floatation another technique again consists in using floating leaves which are assembled together in the manner of a roller shutter.
None of these known techniques gives complete satisfaction. In fact these techniques are either expensive, or need delicate operations for the deployment and retraction of the cover, or are ineffective as regards maintaining the temperature of the water.